ZCS version? (http://www.zimbra.com/forums/announc...r-profile.html)
Relevant sections from /opt/zimbra/log/mailbox.log (& possibly zmmailboxd.out)?
You should have 7 days of logs in that folder by default if that's far enough back to 'last week'.
-At the very least if it happens again hit the drop down arrow & get some of the full error.
-I haven't seen this on the 5.0.x series at all but vaguely remember some 4.5.x bugs.